In the case of road-type vehicles the change in direction of the vehicle which occurs when one of the wheels strikes a hump or a hollow, is relatively insignificant and usually requires no corrective action. The reason for this is that improved roadways, unimproved roadways, and even tilled fields are sufficiently smooth so that any humps and hollows are minor and so quickly traversed that the driver does not feel called upon to make a steering correction.
In conventional designs of heavy off-road earthmoving vehicles it has been the practice to employ roll type suspensions having the same general characteristics as those used in road-type vehicles. More specifically it has been the practice in the design and construction of scrapers, haulers, graders, off-highway trucks, tractors, front end loaders and heavy duty carriers to provide a hinge type connection between the front and rear portions of the vehicle frame, or between the frame and the front axle assembly, which limits relative rolling movement to that about a horizontal axis. Thus in a common type of scraper in which the tractor and trailer portions are integrated into a single articulated frame having rolling rigidity, and in which there is a front axle assembly capable of independent rolling movement, the axis of rolling movement is oriented horizontally both in sprung and unsprung versions. When a major hump or hollow is encountered by one of the front wheels of the vehicle, the lateral tilting of the axle assembly causes the connection between the axle assembly and the frame to shift to the right or to the left. The front end of the vehicle frame is constrained to follow this shifting movement whereas the rear end of the frame is not laterally shifted, with the result that the vehicle frame, incident to ascending a hump, changes its steering direction even though the vehicle steering wheel is maintained stationary. A change occurs in the opposite direction as the wheel descends the hump. In the case of a conventional road vehicle encountering a small irregularity at road speed, the effect of this change in steering direction is momentary, transient, and hardly noticeable. However, in off-road vehicles such as heavy duty scrapers, humps and hollows encountered in normal usage are large and traversed at slow speed so that there is time for the driver to sense an unwanted change in steering direction, both in the ascent and descent of a hump, and to respond to it by the taking of corrective action with the steering wheel.
It can be shown that, using a conventional roll-type suspension, this unwanted change in steering direction also occurs when a rear wheel strikes a hump or hollow, the front and rear wheels normally being acted upon in succession. In either event, the correction which the driver tends to make upon descent is substantially equal to the correction made during ascent, but in the opposite direction, so the corrections nullify one another and there is no net change in direction. The result is that when a driver of an off-road earthmoving vehicle drives over the endless series of humps and hollows which form his "right of way," at usual slow earth-moving speed, he tends to make a corresponding series of corrections in vehicle direction by constant turning of the steering wheel back and forth through a small angle, in a constant effort to direct the vehicle along a straight path. This not only subjects the operator to unnecessary strain and fatigue, but the constant working of the powersteering mechanism, provided on almost all off-road vehicles, results in unnecessary wear and tear and risk of failure of the steering mechanism in the field. The resulting maintenance and down time both involve a large element of expense.